A liquid crystal display (LCD) utilizes the optical and electrical anisotropy of liquid crystal molecules to produce an image. The liquid crystal molecules have a particular passive orientation when no voltage is applied thereto. However, in a driven state, the liquid crystal molecules change their orientations according to the strength and direction of the driving electric field. A polarization state of incident light changes when the light transmits through the liquid crystal molecules, due to the optical anisotropy of the liquid crystal molecules. The extent of the change depends on the orientation of the liquid crystal molecules. Thus, by properly controlling the driving electric field, orientations of the liquid crystal molecules are changed and a desired image can be produced.
The first type of LCD developed was the TN (twisted nematic) mode LCD. Even though TN mode LCDs have been put into use in many applications, they have an inherent drawback that cannot be eliminated; namely, a very narrow viewing angle. By adding compensation films on TN mode LCDs, this problem can be ameliorated to some extent. However, the cost of the TN mode LCD is increased. Therefore, MVA (multi-domain vertical alignment) mode LCDs have recently been developed. In MVA mode LCDs, each pixel is divided into multiple domains. Liquid crystal molecules of a pixel are vertically aligned when no voltage is applied, and are inclined in different directions according to the domains when a voltage is applied. Thus MVA mode LCDs can provide wide viewing angles. Typical MVA mode LCDs have four domains in a pixel, and employ protrusions and/or slits at the pixels to achieve the desired inclinations of the liquid crystal molecules.
Referring to FIG. 3, a cross-sectional view of part of a typical MVA LCD 1 is shown. The LCD 1 includes a first substrate 11, a second substrate 12 parallel to the first substrate 11, and a liquid crystal layer 13 interposed between the first substrate 11 and the second substrate 12. The liquid crystal layer 13 is made of anisotropic liquid crystal material, and has a negative specific inductive capacity (SIC).
A first polarization film 15 is disposed on an outer surface (not labeled) of the first substrate 11 farthest from the liquid crystal layer 13. A color filter 18 is formed on an inner surface of the first substrate 11 nearest to the liquid crystal layer 13. A common electrode 19 is formed on the color filter 18. A plurality of protrusions 111 are formed on the common electrode 19. A first alignment film 119 covers the common electrode 19 having the protrusions 111.
A second polarization film 16 is disposed on an outer surface (not labeled) of the second substrate 12 farthest from the liquid crystal layer 13. A polarization axis of the first polarization film 15 is perpendicular to a polarization axis of the second polarization film 16. A plurality of thin film transistors (TFTs) (not labeled) and a plurality of pixel electrodes 128 connected to the TFTs are formed on the second substrate 12 generally adjacent to the liquid crystal layer 13. A second alignment film 120 covers the TFTs and the pixel electrodes 128.
Each TFT includes a gate electrode 121, a gate insulating layer 122, a semiconductor layer 123, a source electrode 124, a drain electrode 125, and a passivation layer 126. The gate electrode 121 is formed on the second substrate 12. The gate insulating layer 122 is formed on an entire surface of the second substrate 12 having the gate electrode 121. The semiconductor layer 123 is formed on the gate insulating layer 122, and corresponds to the gate electrode 121. The source electrode 124 and the drain electrode 125 are formed on the semiconductor layer 123. The passivation layer 126 is formed on the gate insulating layer 122, the source electrode 124, the semiconductor layer 123, and the drain electrode 125. The pixel electrode 128 is formed on the passivation layer 126, and is connected to the drain electrode 125 via a contact hole (not labeled) of the passivation layer 126. The pixel electrode 128 has a plurality of recesses 129 corresponding to the plurality of protrusions 111.
When no voltage is applied, the LCD 1 is in an off state, and most of liquid crystal molecules 130 of the liquid crystal layer 13 are aligned perpendicular to the first and second substrates 11, 12. When light beams transmit through the second polarization film 16, only the light beams having the same polarization direction as the second polarization axis can pass through the second polarization film 16. When these light beams transmit through the liquid crystal layer 13, a polarization direction of the light beams is parallel to long axes of the liquid crystal molecules 130, and a polarization state of the light beams does not change. When the light beams transmit through the first polarization film 15, all the light beams are absorbed because the polarization direction of the light beams is perpendicular to the polarization axis of the first polarization film 15. Thus, the LCD 1 is in a black state.
Referring also to FIG. 4, when a voltage is applied, the LCD 1 is in an on state, and an electric field is generated in the liquid crystal layer 13. In general, a direction of the electric field is perpendicular to the two substrates 11, 13. In addition, electric field lines of the electric field at two sides near the protrusions 111 and the recesses 129 are arcuate and symmetrical. Because the liquid crystal molecules 130 are anisotropic and have a negative specific inductive capacity (SIC), most of the liquid crystal molecules 130 are twisted such that long axes of the liquid crystal molecules 130 are perpendicular to the directions of the electric field. When light beams transmit through the liquid crystal layer 13, the light beams are birefracted, and thus polarization states of the light beams change. When the light beams transmit through the first polarization film 15, some light beams can pass through the first polarization film 15, and thus the LCD 1 works in an on state.
In the on state, the protrusions 111 and the recesses 129 cause the directions of the electric field to be various, so that the range of different orientations of the liquid crystal molecules 130 is increased. However, the number of protrusions 111 and recesses 129 is limited because of limitations inherent in the structure of the LCD 1 and the technology used in manufacturing the LCD 1. The limited number of protrusions 111 and recesses 129 means the variety of orientations of the liquid crystal molecules 130 is also limited. The LCD 1 has only a relatively small number of domains. Therefore, when a user views the LCD 1 from various directions, the user is liable to see color shift in the images displayed.
What is needed, therefore, is an MVA LCD that can overcome the above-described deficiencies.